Identification and characterization of spiralin-like protein SLP25 from Spiroplasma eriocheiris

Eriocheir sinensis vesicle-associated membrane protein can enhance host cell phagocytosis to resist Spiroplasma eriocheiris infection

Vesicle-associated membrane protein (VAMP) belongs to the receptor protein on the membrane of the secretory transport vesicle and involves in host immune function. The intracellular pathogen Spiroplasma eriocheiris could cause Eriocheir sinensis tremor disease. In a previous study, it was found E. sinensis VAMP (EsVAMP) was differently expressed in S. eriocheiris infection by proteomics analysis. This study mainly aims at the function of EsVAMP in the process of the S. eriocheiris infection. The length of EsVAMP gene was 1681 bp, which contained a 395 bp open reading frame, 90 bp 5'-non-coding region (UTR) and 1277 bp 3'-UTR. The results of qPCR showed that EsVAMP was expressed highly in hemocytes and nerves, followed by gills, intestines and hepatopancreas, and lowly expressed in heart and muscles. EsVAMP in hemocytes was up-regulated after S. eriocheiris infection. After EsVAMP over-expression and S. eriocheiris infection, the RAW264.7 cell morphology and cell viability of the experiment group were significantly better than the control group. Meanwhile, the copy number of S. eriocheiris in the experiment group was significantly lower than that in the control group. After EsVAMP and pCMV-Cre-mCherry were ligated and transfected into RAW264.7 cells, it was found that EsVAMP and lysosome co-localized. Meanwhile, the phagocytosed inactivated S. eriocheiris number and phagocytosed efficiency in RAW264.7 cells were increased significantly. The interference experiment was carried out by synthesizing EsVAMP dsRNA to verify that the EsVAMP transcriptions were successfully suppressed. The S. eriocheiris copy number and the mortality of crab increased significantly after EsVAMP RNAi and S. eriocheiris infection. Meanwhile, the phagocytosed inactivated S. eriocheiris number and phagocytosed efficiency in hemocytes decreased significantly after EsVAMP RNAi and S. eriocheiris infection. These results showed that VAMP was involved in the cell phagocytosis to resist pathogen infection.

Spiroplasma eriocheiris Enters Drosophila Schneider 2 Cells and Relies on Clathrin-Mediated Endocytosis and Macropinocytosis

Spiroplasma eriocheiris causes great economic losses in the crustacean aquaculture industry. However, the mechanism of S. eriocheiris infecting host cells has been poorly studied. We established a Spiroplasma-infected Drosophila Schneider 2 (S2) cell model and investigated its pathogenic mechanism. First, S. eriocheiris induced S2 cell apoptosis and necrosis, seriously decreased cell viability, and increased the production of intracellular reactive oxygen species. Further research showed that S. eriocheiris can invade S2 cells, and the number of copies of intracellular spiroplasmas is sharply increased by 12 h postinfection. In addition, S. eriocheiris can cause S2 cells to form typical inclusion bodies and exhibit large vacuoles. Second, S. eriocheiris is internalized into S2 cells and strongly inhibited through blocking clathrin-mediated endocytosis using chlorpromazine and dynasore. Inhibitors of macropinocytosis, protein kinase C and myosin II, cause a significant reduction in S. eriocheiris in S2 cells. In contrast, disruption of cellular cholesterol by methyl-β-cyclodextrin and nystatin has no effect on S. eriocheiris infection. These results suggest that the entry of S. eriocheiris into S2 cells relies on clathrin-dependent endocytosis and macropinocytosis, but not via the caveola-mediated endocytic pathway. In addition, the intracellular numbers of S. eriocheiris are dramatically reduced after S2 cells are treated with cytoskeleton-depolymerizing agents, including nocodazole and cytochalasin B. Thus, cellular infection by S. eriocheiris is related to microtubules and actin filaments. This research successfully shows for the first time that S. eriocheiris can invade Drosophila S2 cells and provides a process for S. eriocheiris infection.

Production and application of polyclonal and monoclonal antibodies against Spiroplasma eriocheiris

A new species of spiroplasma, Spiroplasma eriocheiris (S. eriocheiris), was identified as a lethal pathogen of tremor disease (TD) in Chinese mitten crab recently. In order to acquire appropriate biological and diagnostic tools for characterizing this newly discovered pathogen, 5 monoclonal antibodies (mAbs) and a polyclonal antibody (pAb) against S. eriocheiris were produced. Among the mAbs, 6F5, 7C8 and 12H5 lead to the deformation of S. eriocheiris. A peptide sequence, YMRDMQSGLPRY was identified as a mimic motif of MreB that is the cell shape determining protein of S. eriocheiris interacting with 3 mAbs. Furthermore, a double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) for detection of S. eriocheiris was established using the mAb and pAb we prepared. It detected as low as 0.1 μg/mL of S. eriocheiris. No cross-reaction was observed with three other common bacteria (Pseudomonas aeruginosa, Escherichia coli, and Bacillus subtilis) and the hemolymph samples of healthy Eriocheir sinensis. Collectively, our results indicated that the mAbs and pAb we prepared could be used in the analysis of S. eriocheiris membrane proteins mimotope and development of a diagnostic kit for S. eriocheiris infections.

Complete genomes of two dipteran-associated spiroplasmas provided insights into the origin, dynamics, and impacts of viral invasion in spiroplasma

Spiroplasma is a genus of wall-less, low-GC, Gram-positive bacteria with helical morphology. As commensals or pathogens of plants, insects, ticks, or crustaceans, they are closely related with mycoplasmas and form a monophyletic group (Spiroplasma–Entomoplasmataceae–Mycoides) with Mycoplasma mycoides and its relatives. In this study, we report the complete genome sequences of Spiroplasma chrysopicola and S. syrphidicola from the Chrysopicola clade. These species form the sister group to the Citri clade, which includes several well-known pathogenic spiroplasmas. Surprisingly, these two newly available genomes from the Chrysopicola clade contain no plectroviral genes, which were found to be highly repetitive in the previously sequenced genomes from the Citri clade. Based on the genome alignment and patterns of GC-skew, these two Chrysopicola genomes appear to be relatively stable, rather than being highly rearranged as those from the Citri clade. Phylogenetic analyses suggest that the susceptibility to plectroviral invasion probably originated in the common ancestor of the Citri clade or one of its subclades. This susceptibility may be attributed to the absence of antiviral systems found in the Chrysopicola clade. Using the virus-free genomes of the Chrysopicola clade as references, we inferred the putative viral integration sites in the Citri genomes. Comparisons of syntenic regions suggest that the extensive viral invasion in the Citri clade promoted genome rearrangements and expansions. More importantly, the viral invasion may have facilitated horizontal gene transfers that contributed to adaptation in the Citri clade.